Wireless data communications networks are often subject to slow fading. Slow fading occurs when the signal propagation environment changes slowly relative to the data communications rate. This may occur when the coherence time of a communications channel is larger than several frame durations.
Slow fading may cause communications problems, such as bursts of errors. Such bursts may significantly degrade data communications performance and negatively impact the reliable decoding of the transmitted data. One approach to mitigating slow fading in wireless networks involves using cooperative diversity techniques, where terminals sharing their antennas and other resources. Such techniques create a virtual array (i.e., a virtual multiple-input multiple-output (MIMO) system) through distributed transmission and signal processing. As a result, spatial diversity and distributed array gains may be realized.
If the communication system can tolerate a certain amount of delay, the erroneous data can be retransmitted using an automatic repeat request (ARQ) protocol. According to such protocols, frames are examined at a receiving device for errors (e.g., through a cyclic redundancy check (CRC)). If no errors are detected in a received frame, the receiving end sends an acknowledgement (ACK) of successful transmission to the sending device. However, if errors are detected, a negative acknowledgement (NACK) requesting retransmission is sent to the sending device. The NACK may not be sent in some cases, for example, if the frame was lost in its way and the receiver did not receive it at all. Also, a sending device may construe a NACK by not receiving an ACK within a time limit. In response to a NACK, the sending device retransmits the frame. This may occur until an ACK is received or until a retransmission number limit is reached.
In addition to ARQ protocols, hybrid ARQ (HARQ) algorithms may be employed. The HARQ protocols operate in a fundamentally similar way with ARQ protocols, but may also combine error detection and correction in order to obtain better reliability and throughput, e.g., data and CRC bits may be additionally protected by an error correcting code which increases the probability of successful transmission. Moreover, HARQ protocols may combine previously observed erroneous packets with the retransmitted packets using techniques such as chase combining, incremental redundancy or code combining.
In certain situations, existing ARQ and HARQ protocols are not sufficient to enhance communication link reliability. For example, when a deep fade occurs, existing ARQ and HARQ protocols may not improve the quality of a communications link, since, poor channel conditions are likely to be experienced by both initially-sent transmissions subsequent retransmissions. Thus, a substantial increase in the average number of retransmissions, as well as a substantial decrease in throughput, may occur.